In so-called "fly-by-wire" aircraft, control surfaces on the aircraft airfoils are not directly mechanically coupled to the controls operated by the pilot. Rather, the couplings are via electrical and/or hydraulic circuits. Needless to say, in order for such couplings to be operative, electrical energy and/or hydraulic fluid under pressure must be available at all times. If there is a failure in an electrical generating system or in a hydraulic pump, the link between the controls and the control surfaces is lost and the aircraft can no longer be controlled.
In the usual case, electrical energy and/or pressurized hydraulic fluid is supplied by one or more pumps or generators driven by the turbine engines used for propulsion of the aircraft, either by thrust or by rotating airfoils. Such turbine engines are equipped with a so-called "AMAD" which is an airframe mounted accessory drive unit. Should the engine, or engines in the case of multiple-engine aircraft, flame out, accessories such as the hydraulic pumps and/or electrical generators driven through the AMAD are no longer driven and control of the aircraft will be lost for the reasons stated.
To avoid this problem, various sorts of emergency power unit systems have been proposed. The purpose of such systems is to provide electrical and/or hydraulic power in the event of an emergency wherein power is not available from the principal power source. The emergency power then provides a link between the control surfaces and the controls allowing the pilot of the aircraft to recover control. Having once recovered control of the aircraft, the pilot may then go about restarting the main propulsion engine or engines.
At the same time, weight in airborne systems is always a concern with every effort being made to minimize weight so as to maximize range and/or payload of the aircraft. Turbine engines used for propulsion, like other engines, require some means of starting them. Not infrequently, a so-called ATSM is connected to each AMAD. An ATSM is an air turbine starter motor and typically includes a turbine wheel connected to the AMAD through which it may drive the main propulsion turbine up to a sufficient speed where it may maintain its own operation. In the usual case, the ATSM is driven by compressed air from any suitable source. Typically, a ground based compressor system such as a so-called ground cart may provide the compressed air. Alternatively, where multiple engines are utilized, the compressed air may be supplied by cross bleed from an already operating engine.